Vehicle windshield wiper systems often stop the wipers, at the end of the wipe cycle, in a position that is lower than the normal inwipe position, generally called a depressed park position. The park mechanisms that provide for the depressed park position generally do so by selectively varying the effective length, although not the actual length, of one link in the linkage that drives the wipers. A different effective length for the variable link translates into a lower inwipe, or park, position for the wipers. The variable length link is typically the link that is directly attached to the motor drive shaft, generally called the crank arm. The effective length of the crank arm is varied by latching it in one fixed angular position relative to the drive shaft during the normal wipe cycle, then freeing it to shift eccentrically to a different angular position when it is desired to park the wipers, which gives a longer effective length. Such park mechanisms may be broadly categorized into two types. Each type needs some mechanism to lock the variable length link during normal wiping, and to then free it to park.
In the first general category of park mechanism, the motor is unidirectional. A latch mechanism fixes the variable link at one angular position until it is desired to end the wipe cycle and park the wipers. Then, a switch activates an external mover, such as a solenoid, to deactivate the latch mechanism and allow the variable link to eccentrically shift to another angular position as the motor continues to coast in the same direction. The inevitable drawback to such a system is the expense and complexity of providing the external mover.
In a second general category, the drive motor is bi directional, that is, reversible. A latch locks the variable link at one angular position during the regular wipe cycle, when the motor rotates in the forward direction. Reversing the drive motor acts to release the latch and allow the variable link to eccentrically shift to a different angular position and a longer effective length. The basic advantage of a reverse to park mechanism is that it is relatively easy and inexpensive to simply reverse the drive motor, compared to providing an external mover. A disadvantage is that, if the latch is designed to be very secure and positive during the normal wipe cycle, it is consequently difficult to release. Conversely, if the latch is made to be easily releasable in response to motor reversal, it is generally not very positive or secure during the regular wipe cycle. Known reverse to park mechanism latches include direction sensitive spring detents, torsional brake springs, one way clutches, simple tension springs, and lost motion pins and slots combined with return springs. None of these are generally as secure and positive as an externally moved latch. Furthermore, structures like direction sensitive spring detents, which depend on friction as a retention force, degrade significantly over time with wear.